Thermoplastic olefin polymers, such as linear polyethylene, polypropylene, and olefin copolymers, such as ethylene propylene copolymer, are conveniently formed in continuous loop-type polymerization reactors and thermoformed to arrive at granules or pellets of the polymers. For example, polypropylene and/or propylene/ethylene copolymers are polymerized in continuous polymerization reactors in which the monomer stream is introduced into a reactor and circulated with an appropriate catalyst to produce the olefin homopolymer or copolymer. The polymer is withdrawn from the catalyst reactor and subjected to appropriate processing steps and then extruded as a thermoplastic mass through an extruder and die mechanism to produce the polymer as a raw material in particulate form, usually as pellets or granules. The polymer particles are ultimately heated and processed in the formation of the desired end products.
Polypropylene and propylene copolymers, as used in various applications involving production of films, fibers, and similar products in the polymers, are thermo-processed and shaped or oriented by one uni-directional or bi-directional stresses. Such polymers are thermoplastic crystalline polymers. Polymers of this nature are subject to degradation due to high temperatures and photochemical action induced by electromagnetic radiation in the visible light range and in the ultraviolet region. In order to retard the degradation of such polymeric objects, the base polymer system, which is molded or extruded to form the desired object, e.g. fiber or film, may be treated with hindered amine light stabilizers, identified by the acronym "HALS," which function to protect the film, fiber, or other object against degradation due to electromagnetic radiation by radiation in the visible light spectrum.
Isotactic polypropylene is conventionally used in the production of fibers in which the polypropylene is heated and then extruded through one or more dies to produce a fiber preform which is processed by a spinning and drawing operation to produce the desired fiber product. The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula: ##STR1## Stereoregular polymers, such as isotactic and syndiotactic polypropylene, can be characterized in terms of the Fisher projection formula. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene, as shown by Formula (2), is described as follows: ##STR2## Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the polymer chain lie on alternate sides of the plane of the polymer. Using the Fisher projection formula, the structure of syndiotactic polypropylene can be shown as follows: ##STR3## The corresponding syndiotactic pentad is rrrr with each r representing a racemic diad. Syndiotactic polymers are semi-crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. Catalysts that produce syndiotactic polypropylene are disclosed in U.S. Pat. No. 4,892,851. As disclosed there, the syndiospecific metallocene catalysts are characterized as bridged structures in which one Cp group is sterically different from the others. Specifically disclosed in the '851 patent as a syndiospecific metallocene is isopropylidene(cyclopentadienyl- 1-fluorenyl) zirconium dichoride.
In most cases, the preferred polymer configuration will be a predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins can be characterized as falling in two general classes, metallocene catalysts and so-called "conventional" Ziegler-Natta catalysts. The conventional Ziegler-Natta catalysts are stereospecific complexes formed from a transition metal halide and a metal alkyl or hydride. Metallocene catalysts are coordination compounds or cyclopentadienyl groups coordinated with transitional metals through .pi. bonding.
The polymerization catalysts may be characterized as supported catalysts or as unsupported catalysts, sometimes referred to as homogeneous catalysts. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components. Traditional supported catalysts are the so-called "conventional" Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride, as disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. A supported catalyst component, as disclosed in the Myer '718 patent, includes titanium tetrachloride supported on an "active" anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Myer '718 is employed in conjunction with a co-catalyst such and an alkylaluminum compound, for example, triethylaluminum (TEAL). The Myer '717 patent discloses a similar compound which may also incorporate an electron donor compound which may take the form of various amines, phosphenes, esters, aldehydes, and alcohols.
Stereospecific metallocenes are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes which may be characterized by the following formula: EQU R"(C.sub.5 (R').sub.4).sub.2 HfQp (4)
In Formula (4), (C.sub.5 (R').sub.4) is a cyclopentadienyl or substituted cyclopentadienyl group, R' is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and R" is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1-20 carbon atoms and p is 2.
Metallocene catalysts, such as those described above, can be used either as so-called "neutral metallocenes" in which case an alumoxane, such as methylalumoxane, is used as a co-catalyst, or they can be employed as so-called "cationic metallocenes" which incorporate a stable non-coordinating anion and normally do not require the use of an alumoxane. For example, syndiospecific cationic metallocenes are disclosed in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, the metallocene cation is characterized by the cationic metallocene ligand having sterically dissimilar ring structures which are joined to a positively-charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Similar relationships can be established for isospecific metallocenes.
While metallocene catalysts are generally proposed for use as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described in the Welborn '432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, zirconia and the like. Non-metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The Welborn '561 patent discloses a heterogeneous catalyst which is formed by the reaction of a metallocene and an alumoxane in combination with the support material. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a "conventional" supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamshoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. Nos. 5,308,811 to Suga et al and 5,444,134 to Matsumoto.
The polymers normally employed in the preparation of drawn polypropylene fibers are normally prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in the aforementioned patents to Myer et al. U.S. Pat. Nos. 4,560,734 to Fujishita and 5,318,734 to Kozulla disclose the formation of fibers by heating, extruding, melt spinning, and drawing from polypropylene produced by titanium tetrachloride-based isotactic polypropylene. Particularly as disclosed in the patent to Kozulla, a preferred isotactic polypropylene for use in forming such fibers has a relatively broad molecular weight distribution (abbreviated MWD), as determined by the ratio of the weight average molecular weight (M.sub.w) to the number average molecular (M.sub.n) of about 5.5 or above. As disclosed in the Kozulla patent, the preferred molecular weight distribution, M.sub.w /M.sub.n, is said to be at least 7.
It is also known to produce polypropylene-based fibers from syndiotactic polypropylene. Thus, as disclosed in U.S. Pat. No. 5,272,003 to Peacock, syndiotactic polypropylene, such as that produced by syndiospecific metallocenes of the type disclosed in the aforementioned U.S. Pat. No. 4,892,851, can be used to produce polypropylene fibers using various techniques disclosed therein and identified as melt spinning, solution spinning, flat film spinning, blown film, and melt blowing or spunbond procedures. As disclosed in Peacock, the syndiotactic polypropylene, as characterized by polymer configuration, comprises racemic diads connected predominantly by meso triads. As noted in Peacock, the syndiotactic polypropylene fibers may be in the form of continuous filament yarn, monofilaments, staple fiber, tow, or top. Syndiotactic fibers, as thus produced, are characterized as having substantially greater retraction value than fibers formed of isotactic polypropylene. This enhanced elasticity is said to form an advantage of the syndiotactic polypropylene fibers over isotactic polypropylene fibers for use in garments, carpets, tie downs, tow ropes, and the like.
In employing polypropylene and other polyolefin polymers, there are large numbers of additives which are sometimes used. As noted previously, hindered amine light stabilizers can be used. Other stabilizers which can be employed to stabilize the polymer product and the products into which the polymers are incorporated include various additives which function to stabilize the polymer products, not only against light or UV degeneration but also against thermal or oxidative degeneration or actinic degradation. Thus, as disclosed in U.S. Pat. No. 4,325,863 to Hinsken et al, various benzofuranone or indolinone compounds can be used as stabilizing agents for such polymers as polypropylene, polyethylene, propylene ethylene co-polymers, and various other polymeric materials. As disclosed in Hinsken et al, a wide range of benzofuranones, which may be substituted or unsubstituted to include both polycyclic and monocyclic lactones, can be employed. Preferably, these are incorporated in extruders in which the stabilizing compounds are mixed with granules of such polyethylene or polypropylene granules and then extruded into the desired product. U.S. Pat. No. 5,175,312 to Dubs et al discloses the use of various phenol benzofuran-2-ones as stabilizers in the polymeric materials which are formed into films, fibers, tapes, and the like. The 3-phenol benzofuranones of Dubs et al are generally characterized to include 3-phenol benzofuranones with substituent groups at the 7 position of relatively high molecular weight, e.g. alkyl, containing 14 or more carbon atoms, with substituent groups at either or both of the 4 and 5 positions of alkyl groups or various cyclo-alkyl substituents of somewhat lower molecular weights. The phenol substituent at the 3 position can be substituted with C.sub.1 -C.sub.4 alkoxy, halide, or C.sub.1 -C.sub.8 alkyl groups.